The easiest way I find is to put a CD in the drive then click on the CD/DVD icon from the list. The tracks will then appear in the 'tray' You then double click or drag them into the storyline mode at the bottom.
jackiet147 is right, that's easy. However, every time you load your slideshow to work on it, you will have to load the same CD in the same drive first. Otherwise, you will get an error message saying the music file is not available. You are better off using music already on your hard drive, and dragging this file to the timeline.